Field of the Invention
The invention relates to a method for the production of a flame ionization detector, whereby the flame ionization detector has at least one combustion chamber, at least one channel for directing a gas or gas mixture, and at least one electrode structure. The invention also relates to a corresponding flame ionization detector.
Description of Related Art
Flame ionization detectors (FID) are used for the measurement of organic compounds, in particular volatile hydrocarbon-containing substances. The principle of operation consists in that the electrical conductivity of an oxy/hydrogen flame between two electrodes is measured. The medium that is to be analyzed is in this case directed into a combustion space with a combustion gas (usually hydrogen gas or a mixture of hydrogen and helium) and an oxidizing agent (in most cases oxygen or air) and ionized there. The ions are detected by a voltage being applied to two electrodes (measuring electrode and counter-electrode), which are arranged in the area of the combustion chamber, and the ion stream that is produced is measured and evaluated.
One advantage of the flame ionization detectors consists in the fact that the measurement signal over a wide concentration range is linearly proportional to the amount of the analytes, or more precisely to their hydrogen-bound carbon content. Therefore, for example, the concentration of a hydrocarbon can be determined without prior calibration.
A flame ionization detector usually has a combustion chamber, electrodes, channels for feeding in and drawing off gases or gas mixtures, and an ignition device. The openings of the channels into the combustion chamber are referred to as nozzles. In order to prevent leakage currents flowing through the system from being co-detected and thus distorting the measurement, in most cases a protective electrode is also provided.
If the combustion gas and the oxidizing agent are directed into the combustion chamber from opposite directions, the arrangement is also referred to as a counter-current burner. Counter-current flames burn in the immediate vicinity of a stagnation point, which because of the minimal heat loss accounts for the high ionization efficiency of such flames.
Since hydrogen is used in the measurement, flame ionization detectors are preferably configured to be small, so as to reduce the risk of explosion. Such a miniaturized flame ionization detector is referred-to as a μFID.
Flame ionization detectors that are produced with the method of the microsystem technique are known. The International Patent Application Publication WO 2009/036854 A1 or the related European Patent EP 2 191 263 B1 and corresponding U.S. Patent Application Publication 2010/0301870 A1, describe a flame ionization detector that is designed to be planar and that consists of at least three small plate-like substrates that consist of glass or silicon and that are connected to one another.
The German Patent Application DE 10 2009 035 762 A1 and European Patent Application EP 2 447 716 A1 describe a flame ionization detector that also consists of three layers (glass or silicon) and that is manufactured by means of the method of the microsystem technique, and said flame ionization detector is configured as a counter-current burner. The measuring and protective electrodes are applied in a thin-film technique to the bottom of a substrate, whereby the side walls of the combustion chamber form the counter-electrode. Moreover, a temperature-dependent resistor is also provided as a temperature sensor. Different arrangements of the channels for the directing of gases are discussed. Based on the fact that substrates that consist of different materials are used with temperature coefficients that are different to some extent, the detector can be adversely affected by thermal stress depending on the application, or the structures of the detector can be configured based on different expansion behavior as a result of temperature changes.
In the article “Development and Analysis of a LTCC Micro Stagnation-Point Flow Combustor” by Ming-Hsun Wu and Richard A. Yetter, J. Micromech. Microenc. 18 (2008), Number 12, a counter-current burner is described, which was produced by means of an LTCC method.
Low-temperature co-fired ceramics (LTCC) are used to produce multi-layer ceramic structures. In this case, unfired, so-called “green” ceramic films are structured individually, stacked, laminated, and subjected to a sintering profile at a peak temperature of approximately 850° C.-900° C.; below, the technical term “green ceramic films” is used to describe unfired ceramic films. At the maximum temperatures that occur during sintering, the LTCC method is distinguished from the production of high-temperature co-fired ceramics (High-Temperature Co-Fired Ceramics, HTCC), which are sintered at temperatures of between 1600° C. and 1800° C. In addition, thick-layer hybrid techniques are known, whereby strip conductors or resistors are applied using the silk-screen method on already-sintered ceramic substrates. The printed carrier is fired, whereby the applied pastes fuse to the layers. Subsequently, assembly of the discrete components takes place.